1. Field of the Invention
The present invention relates to a system for improving DSL transmission rates over standard telephone lines. More specifically, the present invention is directed to a device and method that reduces the effect of echos/reflections from bridgetap telephone lines to thereby improve the DSL transmission over a standard telephone line.
2. Discussion of Background Information
Homes and businesses in the U.S. connect to local telephone providers through a network of wire connections, many of which are underground. To provide alternative uses for the lines, many of these connections include “bridgetap” lines, which include a pair of wires spliced into the main line that extend away from the main line. An example of such a circuit is shown in FIG. 1. A Central Office (“CO”) frame 100 connects through a pair of tip and ring wires 101 and 102 (typically 19-26 gauge wires, preferably 24-26) to a cross connect box 104. Tip and ring wires 101 and 102 continue from box 104 underground to a first terminal 106 at a user's location, to which a standard telephone can be connected. A first end of a bridgetap 110, which includes a pair of tip and ring wires 112 and 114, electrically connect in parallel with tip and ring wires 101 and 102 at points A and B, respectively. A second end of bridgetap 110 terminates as an open circuit some distance away (based on the length of the bridgetap wires).
When the tip and ring wires 101 and 102 are installed underground, the connection points A and B at the first end of bridgetap 110 are also typically underground. The opposite end of bridgetap 110 is typically above-ground at a telephone junction 116, with the intention of having such ends available to connect to another terminal.
By its very nature, some of the energy of a transmission over tip and ring lines 101 and 102 will split into the bridgetap 110 and produce an echo signal. However, the effect of such an echo is negligible at the low frequencies of voice communication over a telephone network. Thus, for the many years that such networks have been installed, the value of bridgetaps have far exceeded their drawbacks, such that they can be found throughout the telephone networks of every telephone carrier in the U.S.
In recent years, the increase in Internet use has developed a market for high speed Internet access. One such type of high speed access is a digital subscriber line (“DSL”), which uses the existing telephone lines to transmit data at high frequencies well outside the low frequency voice range. However, the effect of bridgetap echo can be far more significant at such high frequencies, particularly in the downstream direction (i.e., from CO frame 100 to terminal 106). Specifically, when the length of bridgetap 110 approaches the resonance frequency of the bridgetap (i.e., (λ/4)90°), bridgetap 110 becomes the circuit equivalent of a series connection of an inductor and a capacitor, which results in a very low resistance and low impedance. Based on the resulting resonance characteristics, the bridgetap will create/reflect a significant echo in the downstream direction of data transmission. If the connections A and B are within approximately 1000 feet from terminal 106, the echo can significantly decrease the S/N ratio of the transmission, which in turn decreases the downstream data transmission rate.
Based on the resonance characteristics of standard telephone lines, signal degradation will define a generally bell shaped curve between 250-650 feet, with maximum losses between 300 and 500 feet. Tests have shown that, for a connection with a downstream transmission rate of 2.3 Mb/s without bridgetap, the addition of 300 feet of bridgetap cable spliced into the main line 1000 feet from a user reduced the downstream transmission rates from 2.3 Mb/s to between 1.48-1.51 Mb/s. These losses may result in a downstream transmission rate that is below FCC minimums, such that the DSL provider will be forced to deny service to such a user absent corrective measures.
At present, the only way to effectively eliminate the effects of bridgetap 110 is to sever it from the main line near points A and B. Since most bridgetaps are underground, this requires a crew of men and equipment to dig up the lines near connection points A and B to cut the bridgetap off. The assignee of the present invention currently charges approximately $900 for this service. Further, once cut, the bridgetap cannot be used absent reattachment, and essentially becomes waste underground cable.